All-Williams US Open clash is all Serena

On song: Serena Williams on the way to a US Open third-round victory over her sister Venus on Friday

Serena Williams turned her 30th career meeting with her sister Venus into a one-woman showcase Friday, reaching the US Open fourth round with a 6-1, 6-2 victory.

Reluctantly rising to an occasion neither sister enjoys, Serena said she played her best match since she returned in March, more than six months after giving birth to daughter Olympia last September 1.

"I played much better tonight than I have since I started this journey on my way back," said Serena, although she wasn't sure she'd agree with Venus's assessment that it was the best match she'd ever played against her sister.

"I think it's by far the best match I ever played against her in forever," she said. "But I don't know about ever, ever."

Either way, the scoreline equalled Serena's most lopsided victory over Venus, and took her head-to-head record against her sister to 18-12.

"She played so well, I never got to really even touch any balls," Venus said. "I don't think I did a lot wrong. But she just did everything right.

"Obviously that level is definitely where she's going to want to stay during this whole tournament.

The most dramatic moment of the contest came when Serena rolled her right ankle in the second game. She took a medical timeout at the next changeover to have more tape added the bandage she was already wearing under her sock.

There was no sign she was affected as she romped through seven straight games.

"It feels good for the match to be over with. Win or lose, it just feels good that that's done," said Serena, who showed none of her usual ebullience on court even as she belted 34 winners and 10 aces.

When she sealed it with a forehand winner, the sisters met at the net for a hug then walked to their chairs before Serena acknowledged the crowd with a wave.

"It's not easy," Serena said of facing Venus. "She's my best friend. She means the world to me. Every time she loses I feel like I do."

Since falling to Angelique Kerber in the Wimbledon final, Serena had endured erratic results in the hardcourt buildup to the US Open.

But she looked every inch a contender to claim a record seventh US Open crown that would see her equal Margaret Court's record of 24 major titles.

Serena will try to take the next step when she takes on Kaia Kanepi for a place in the quarter-finals.

The Estonian, who toppled world number one Simona Halep in the first round, defeated Sweden's Rebecca Peterson 6-3, 7-6 (7/3).

- Stephens advances -

Defending champion Sloane Stephens, at No. 3 the highest seed left in the draw after the exits of Halep and world number two Caroline Wozniacki, booked her fourth-round berth with a 6-3, 6-4 victory over former world number one Victoria Azarenka.

Up a break in the final set, Stephens had slipped behind 4-3 when a light shower prompted officials to close the Ashe Stadium for the first time this week.

Both players had a chance to leave the court and the break refreshed Stephens, who came back to win the last three games in the "super-cool" atmosphere of the closed stadium.

"It kind of felt like a night match with the lights on and everything," said Stephens, who said the break allowed her to "refocus".

"I played much better the last three games," she said. "I competed. When things got tough, I just hung in there, battled as hard as I could, ran down every ball."

Stephens will face 15th-seeded Belgian Elise Mertens for a place in the quarters after Mertens beat Czech Barbora Strycova 6-3, 7-6 (7/4).

Pliskova, the 2016 US Open runner-up, rallied from a break down in the second set to secure the victory and set up a meeting with 22-year-old Australian Ashleigh Barty.

Barty is through to the fourth round of a Grand Slam for the first time after a 6-3, 6-4 victory over Karolina Muchova, the Czech who toppled two-time Grand Slam winner Garbine Muguruza in the second round.